Transmission circuits



' cuits, and particularl Patented Nov. 23, 1926.;5

UNITED STATES PATENT OFFICE.

WILLIAM L. CASPER, OF BROOKLYN, NEW YORK, ASSIGNOR TO WESTERNE-LECTRICCOMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

TRANSMISSION CIRCUITS.

Application filed August 4, 1922. Serial No. 579,593.

This invention relates to transmission cirto such circuits in whichamplifying devices are employed.

An object of this invention is to prevent an amplifying device fromsinging due, for example, to a portion of the output energy, finding itsway back into the input circuit of the device.

Another object is to reduce the resonance peak in the transmissioncharacteristic curve of an amplifying device, caused by resonanceeffects between the capacity existing between the terminals of theamplifying device external inductance elements.

This invention is hereinafter described in detail in connection with anamplifier circuit working into a load which has a falling frequencytransmission characteristic. As explained in my Patents Nos. 1,530,648and 1,530,649, issued March 24, 1925, the leakage inductance of an inputtransformer of an amplifier or a separate series inductance element ofsuitable value may be made to resonate with capacity effects between theelectrodes of the amplifier to give a transmission characteristic risingwith frequency.

Such a rising characteristic over the transmission band is. frequentlyof importance when the amplifier works into a translating device havinga transmission characteristic falling -with the frequency. This 11S1I1gcharacteristic is frequently obtained by resonating the leakageinductance and the capacity effects at a frequency above the desiredtransmission range, a suitable filter being employed to suppressfrequencies around the resonance frequency, so that the shape andcharacter of the peak doesnot enter-into the transmission characteristicof the repeater system. I In'the absence of such a filter, however, thetransmission at or near the resonance frequency may be so much greaterthan that over the essential range thatthe amplifier. sings or howls atthe resonance frequency-an effect which is, of course, undesirable. Inaccordance with this invention, a simple-and effective way of reducingthis resonance peak to a value comparable to the re mainder of thetransmission curve is by con-. meeting a condenser across two of theelectrodes of the amplifier. This, while lowering the frequency of the.resonance peak, has also been found to reduce to a considerable extentthe height of the peak and the conand I 1 illustrates an amplifiercircuit em bodylng this invention in which the correcting condenser isconnected across the grid and cathode electrodes of the amplifien.

Fig. 2 is a modification of Fig. 1 in which the correcting condenser isconnectedacross the grid and anode electrodes.

Fig. 3 illustrates, by means of curves, the effect of shunting two ofthe electrodes of an amplifier by a condenser.

Fig.1 discloses a vacuum tube amplifier- 10 connectedbetween an incomingline 11, 11 and an outgoing line 12, 12 for impressing upon the outgoingline in am lified form signals impressed upon the tube from incomingline 11. The control electrode 13 and the cathode 14 of the amplifierare connected to the incoming line 11 by an input transformer 15, whilethe anode 16 and cathode 14 are connected to the outgoing line 12 by asuitable output transformer 17. The cathode 14 i's heated by a batteryor other source of volta e 18 and batteries 19 and 20 are employed ormaintaining respectively'the grid and the anode at suitable potentialswith respect to the cathode. e

As has been explained, it is frequently desirable to have a vacuum tuberepeater operating with a transmission characteristic rising withfrequency; One way this may be accomplished is to resonatev the capacityeffecis existing between the electrodes of the tube with 'a smallinductance in 'series with a winding of the input transformer. An actualinductance element separate from the input transformer is not necessarysince it has been found that leakage reactance in an input transformermay be adjusted to function the same as a series inductance element. Forexample, in Fig. 1 the dotted inductances 21 and 22 may represent theeffective series inductance caused by leakage reactance in transformer15. These effective inv ductances maybe made to resonate with the caacity effects between the grid 13 and catho e 14 at a frequency in theupper sneecll. range such as shown by curve23 of Fig. 3

.am lifier expressed in miles wherein the abscissae represent frequencyand the ordinates represent the gain of the of standard cab e. I

This rising transmission characteristic is particularly useful when theamplifier works into a translating device, such as a 'loud speakingreceiver, the transmission equivalent of which has a characteristicfalling with the frequency. The resonance peak of curve 23 howeveroccurs at a frequency which is generally not essential to give quality.

Y transmission an may, for example, occur at resonating frequency, thisinvention provides a condenser 24 connected in shunt to the grid andcathode of the tube. The presence of this condenser lowers theresonating frequency and, at the same time, has been found to ive theresult that the resonance peak is 0% a considerably less magnitude.Curve 25 of Fig. 3 illustrates, for example, the transmissioncharacteristic obtained by employing a condenser 24 of such a value thatthe resonance peak occurs in the neighborhood of 6000 cycles per.second. This curve shows that the resonance peak is of a considerablyless height and also rapidly falls ofi for frequencies above theresonating frequency so as to act substantially as a cutoff filter. Dueto the pre ence of condenser 24Eand the resulting lowering of theresonance peak the amplifier l0 can be worked at greater amplificationwithout danger of singing being produced.

It has been found that substantially the same results may be obtained byconnecting the correcting condenser between the grid and anode. Fig. 2illustrates a condenser 27 connected across the grid 28 and anode 29 ofa vacuum tube repeater 30. This condenser 27 may be made to resonatewith the leakage reactance of the input transformer 31 and the capacityefiects between the elec trodes of tube 30 to give the transmissioncharacteristic illustrated as curve 32 of Fig. 3 whereas the absence ofthe correcting condenser 37 would give a transmission char.- acteristicsimilar to curve 2?.

As a specific example of the value co'ndenser 2a of Fig.1 may have, itmay be stated that in one case where the effective grid cathode capacitywas 100 micromicrm farads and the leakage reactance gave an marywinding, its value will, of course, be

dependent upon theimpedance ratio of the transformer and should beInulti lied by the im. edance ratio to determine t h in uctanceresonating pacity. r

While this invention has been described above in certain specificembodiments, it is to with the shunt caother different forms withoutdeparting in any wise from the spirit of the invention as defined inappended claims.

The invention claimed is:v

e effective be understood that the invention may-possess 1. Thecombination with two transmission lines across one of whichan-appreciable capacity efl'ect exists, and means for coupling saidlines to providesubstantially constant transmission between said linesfor a wide band of frequencies, said capacity efiect tending tocause,the upper frequencies of said band to be,less efiicientlytransmitted than the intermediate frequencies thereof, and means tendinto resonate. with said capacity to substantially equalize thetransmission at said intermediate frequencies and said upper frequencieswhile maintaining the transmission efiiciency for said intermediatefrequencies substantially unchanged at the value caused b said capacityeffect; of a capacity connector across one of said lines andproportioned to lower the frequency at which .said first mentionedcapacity and means tend to resonate.

2. The combination with means comprising a transformer and aspace'discharge de-- vice, said means having a transmission frequencycharacteristic showing high transmission eliiciency for electricalvariations over a wide frequency band, said means including inductanceeffectively in series with a w1nding of said transformer, and saiddcyicc comprising electrodes and an effective input capacit -tending tocausethe upper frequencies 0 said band to be less efficientlytransmitted than the intermediate frequencies thereof, said effectiveinput capacity resonating with said inductance at such a frequency thatthe transmission frequenc characteristic of said means rises with euency'at and above the upper limit of said and and remains substantiallyunchanged for said in termediate frequencies at the value caused by saidefiective input capacity; of a. capacity connected across certain ofsaid electrodes and of such a value that the transmission frequencycharacteristic of said means is lowered at the frequency of said firstmentioned resonance.

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3. The combination with means comprising a transformer and a spacedischarge device, said means having a transmission frequencycharacteristic showing high transmission efliciency for electricalvariations over a wide frequency band, said means including inductanceeffectively in series with a'winding of said transformer, and saiddevice including a cathode and a control electrode between which anappreciable capacity effect exists tending to cause the upperfrequencies of said band to be less efficiently transmitted than theintermediate frequencies thereof,

said capacity resonating with said inductance at such a frequency thatthe transmission frequency characteristic-of said means rises withfrequency at and above the upper limit of said band and remainssubstantially unchanged for said intermediate frequencies of a capacityconnected across said cathodeat the value caused by said capacityeffect;

and control electrodeand of such a value that the transmission frequencycharacteristic of said means is lowered at the frequency of said firstmentioned resonance and maingainad rising with frequency above said 4. Atransmission circuit comprising an incoming line and an outgoing line,an amplifier for repeating from said incoming line to said outgoing lineelectrical variations extending over a wide frequency band, saidamplifier having electrodes between which an appreciablepapacity effectexists,

the transmission eificiency for said intermemediate frequencies remainssubstantially unchanged at the value caused by said capacity effect, anda capacity connected across two of the electrodes of said am lifier andof such a value that-the transmission efliciency of said amplifier islowered at the frequency of said first mentioned resonance.

In witness whereof, I hereunto subscribe quencies and said upperfrequencies while my name this 2nd day of August, A. D. 1922.

WILLIAM L oAsPER.

